Its already possible to buy car's "cigar lighter USB port"
device designed to plug your phone into to charge up on the move. These are
common, and replace your cars lighter.

This means that the simple bit of electronics needed to turn
the cars 12v (or the trucks 24v) into 5v for the USB supply is taken care of
extremely easily and comes complete with the required USB socket for around 3 UK
Pounds.

All that remains is to connect it to your powerchair. But
there's nowhere to plug it in! Well your 3 pin charge socket on your chair is
also an outlet and "live" and
connected to the chairs 24v battery system at all times.

So all we need do is
connect this to the cheap eBay cigar lighter device and we have a safe easy
charger for the phone.

There are loads of these devices on eBay for a very low price,
and most are 12v / 24v so these are suitable. I used a Belkin manufactured one,
simply because I already had it. Almost any will do though. See
here

I also ordered a charge socket connector to fit the
powerchair, which is a Neutrik or XLR connector. These are about £3 UK pounds. Total
outlay £6
here This is the same as the one on your charger.

HOW!

Look at the image on the right.

The BLUE one is the powerchairs charge socket.

The black one is your chargers plug, or your new phone connector XLR / Neutrik
plug.

You need pin 1 and 2 only.

On MOST chairs 1 is positive. So it goes to the centre pin on the car cigar
lighter plug. And pin 2 is negative and goes to the outside.

My HTC phone, its USB charge lead, and the adapter (top left) that converts the
powerchairs charge socket, to a 5v USB phone charge socket. Small enough to keep
in a pocket or your wallet.

Take one of these car cigar lighter USB things apart as below.

The "Belkin" cigar lighter / USB socket designed for a car. £3 from eBay.
Obviously dismantled here! Throw away the shell, cut off the steel "wings" (this
is the negative). The spring in the middle is positive. It is a mini inverter.
It changes 24v to 5v efficiently with nearly no loss or heat.

Here I have already added a 2 core cable to the Neutrik "charger" socket. These are £3
on eBay too. The cable needs soldering, and any old bit of cable will likely do
here.
This is a bit that came from an old hair dryer. My hair is long gone! The bit on
the right is the tiny inverter taken from its case, and changes the 24v to 5v for the USB.

Now has the cable attached. This is the POS side. (the spring.) Its
soldered to, and runs in the spring for added support!

This is the NEGATIVE side. Note the heat shrink tube... Add a much larger size
over the top of the complete thing will protect it when completed. As below.

When you plug in to the powerchair's charge port there will be an LED that lights
up. At least on the Belkin version. This is ready to use. Works great. Its that simple.

DON'T GET THE PLUS AND MINUS MIXED UP! It will fry the circuit board most
likely. And waste £3... If unsure of powerchair charge plug
polarity just test it with a volt meter. But the vast majority are as per the
diagram above. Tiny bit of simple soldering, and £6 worth of bits! Now I can
use sat navigation, display maps on screen, and the phone battery doesn't die.
It charges up instead.

Works for any 24v powerchair. Runs and charges iPhones, HTC, Blackberries, Tablets, or anything that normally needs power
or charges from a USB port.

There's a thread on my forum discussing this
here And contrary to some peoples opinion it will not damage your
powerchair. That socket can supply 8 amps+ Quite safely. If you worry about
these things, its also a good idea to add a 1 Amp (1000mA) fuse in the cable.